Over the years, gas turbine-powered aircraft have used a variety of configurations in achieving vertical or short takeoffs and landings. One method is to vector the engine exhaust from one or more turbojet engines downward to create lift. Once airborne, this type of aircraft gradually transitions the thrust aft until a forward airspeed sufficient to support the aircraft is reached, at which point the aircraft is wing-borne and conventional aerodynamics may take over. Other configurations use remotely located lifting fans powered by compressor stage bleed air from the turbojet engines to create lift. Alternatively, a ducted mixture of compressor bleed air and turbojet exhaust gas may be ducted to remotely located nozzles that discharge a downward thrust, thereby creating reaction-lifting forces that lift and control the aircraft. While these configurations often use very high disc loading fans, they are still more efficient than pure jet variants.
An exemplary high disc loading lift-fan aircraft is the Ryan XV-5, which was developed during the 1960s and flown successfully in 1968. The XV-5 used a pair of General Electric J-85 turbojet engines and three lift fans for controlled flight. Installed in each wing was a 62.5″ diameter fan to provide the majority of the thrust, with a smaller fan in the nose to provide some lift as well as pitch trim. For vertical liftoff, jet engine exhaust was diverted to drive the lift-fan tip turbines via a diverter valve. The core engines provided a total thrust of 5,300 pounds in forward flight mode, but could generate a total lift thrust of 16,000 pounds via the lift fans in hover mode. Using the lift fans provides a 200% increase in the total thrust, a clearly advantageous feature for vertical takeoff and landing (“VTOL”) aircraft.
Unfortunately, a disadvantage associated with ducted lift-fan aircraft is momentum drag, which causes an aircraft to require much higher power levels. Momentum drag is generally caused by a directional change of the airflow going through the lift fans. For instance, the fan flow initially has horizontal momentum (relative to the vehicle and due to the forward speed of the vehicle), but exits vertically, with no relative horizontal momentum. This change in momentum results in a horizontal force towards the back of the vehicle (i.e., momentum drag), which has an effect similar to normal aerodynamic drag. This momentum drag is a function of the mass flow rate of air through the fans times the forward speed of the vehicle through the air. Often the goal of lift-fan aircraft design is to increase the mass flow of air by using larger diameter fans with a smaller lift per unit area. This reduces the power needed to produce the required lift; however, the associated higher mass flow can greatly increase the momentum drag.
As momentum drag can become quite large, forward flight thrusters require a large amount of power to enable the aircraft to fully accelerate to wing-borne flight. Therefore, a need exists for a system and method to both increase efficiency and reduce the momentum drag. More specifically, a need exists for a system and method for increasing efficiency and reducing the momentum drag of a light or low disc loading lift-fan aircraft, such that its peak power requirements are not greatly increased from its hover power requirements.